1. Field of the Invention
The present invention relates to a twin-tube hydraulic damper.
2. Description of the Prior Art
Prior art references disclose solutions providing improvements of acoustic performance of hydraulic dampers. For example, Patent publication DE102008042251 discloses a noise suppression unit arranged on the piston rod formed by a cylindrical sleeve with a diameter lower than diameter of the piston, arranged on the piston compression side. That construction provides annular inertial flow channel between the outer surface of the sleeve and the inner surface of the damper tube. That additional working liquid flow path provides noise suppression effect by means of the liquid inertial force.
Patent publication JPH10-220515 in turn discloses a hydraulic buffer for noise reduction, in which an additional sloped disc is added on a rebound side of the piston assembly specifically in the inlet area of the axial channels. The inclination of said sloped disc provides a vortex flow of working liquid through said axial channels. The vortex flow suppress cavitation produced upstream of the axial channels and thereby reduces the fluid noise.
High frequency vibrations of a piston rod result in rattling (rambling, knocking) noise and if the damper is a part of a vehicle suspension system, these vibrations may be audible even in the passenger compartment. The rattling noise intensity and occurrence obviously depends on a vehicle suspension setup and an acoustic performance of a vehicle chassis. Nevertheless, it occurs mainly at low amplitude, high frequency damper stroke reversals, occurring for example while driving over the road paved with setts (also called Belgian blocks) with relatively low speed of around 20-40 km/h. The most irritating rattling noise occurs at the piston rod vibrations frequency within the range of 300-600 Hz. Poor isolation parameters of the vehicle chassis and the top mount at the end of the piston rod, as well as low level of a background noise related to low vehicle speed at which this rattling noise occurs additionally makes this noise hard to ignore.
The inventors discovered that these vibrations are generated by a deflective disc of a rebound base valve assembly due to pressure fluctuations occurring in particular during rapid changes of the stroke direction.